And an analysis of those archives reveals not only how different can the Sun appear at different wavelengths (a trivial concept, after all) but also how what looks like a sunspot in one image may be pretty much invisible in another.

But if some sunspots appear to be much easier to identify if the “correct” filters are used during observation, wouldn’t a more precise definition of “sunspot” be needed? For example “it appears in at least 3 filters” or “it can be seen in the SOHO MDI channel” or whatever else.

Otherwise when the Sun will wake up later this year or next, the count of sunspots is going to be truly enormous, and comparisons with the past impossible.

Because the blue Calcium K Line (393.3 nm) is sensitive to magnetic fields, magnetically active structures show up in high contrast against the surrounding chromosphere. Places where moderate magnetic fields exist show up bright whereas images of high magnetic fields are dark.

Given that the same thing just happened on Aug 31, 2009, I’m wondering if you could find examples in the archives of similar “sunspots” that were not counted. This might show the apparent bias of wanting to end long streaks of sunspot-less-ness.